Solid-state imaging device and method for manufacturing solid-state imaging device

ABSTRACT

According to one embodiment, a solid-state imaging device is provided. The solid-state imaging device includes a photoelectric conversion element, a floating diffusion, and an amplifying transistor. The photoelectric conversion element photoelectrically convert incident light into electric charges with an amount corresponding to an amount of the incident light, and accumulates the electric charges. The floating diffusion accumulates the electric charges read out from the photoelectric conversion element. The amplifying transistor includes a gate electrode connected to the floating diffusion, and outputs a signal based on the amount of the electric charges accumulated in the floating diffusion. The amplifying transistor includes a first concentration region disposed in at least a part of the maximum region of the depletion layer and a second concentration region disposed at a deeper position than the first concentration region, and has higher impurity concentration than that of the first concentration region.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims the benefit of priorityunder 35 U.S.C. §120 from U.S. Ser. No. 14/105,887 filed Dec. 13, 2013,and claims the benefit of priority under 35 U.S.C. §119 from JapanesePatent Application No. 2013-164363 filed Aug. 7, 2013; the entirecontents of each of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a solid-state imagingdevice and a method for manufacturing the solid-state imaging device.

BACKGROUND

Conventionally, a solid-state imaging device includes a plurality ofphotoelectric conversion elements disposed corresponding to respectivepixels of a captured image. Each photoelectric conversion elementphotoelectrically converts incident light into electric charges of anamount corresponding to the received light intensity and accumulates theelectric charges in a charges storage region.

The solid-state imaging device reads out the electric chargesaccumulated in the charges storage region of each photoelectricconversion element using a read-out transistor and accumulates theelectric charges in a floating diffusion. Subsequently, an image signalis obtained after an amplifying process by an amplifying transistor.

The amplifying transistor amplifies an image signal corresponding to theamount of electric charges accumulated in the floating diffusion (an FDvoltage) and outputs the amplified image signal. Accordingly, the outputof the amplifying transistor is preferred to change linearly withrespect to the FD voltage in order to obtain an appropriate imagesignal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a schematic configuration of adigital camera with a solid-state imaging device according to a firstembodiment;

FIG. 2 is a block diagram illustrating a schematic configuration of thesolid-state imaging device according to the first embodiment;

FIG. 3 is a diagram illustrating one example of a circuit configurationof a pixel array according to the first embodiment;

FIGS. 4A and 4B are graphs illustrating an output characteristic of aconventional amplifying transistor;

FIGS. 5A and 5B are schematic cross-sectional views of the conventionalamplifying transistor;

FIG. 5C is a graph illustrating a relationship between: a substrate-sidedepth of a depletion layer formed in the conventional amplifyingtransistor, and a potential;

FIG. 6 is a schematic cross-sectional view of the pixel array accordingto the first embodiment;

FIG. 7 is a schematic cross-sectional view of an amplifying transistoraccording to the first embodiment;

FIG. 8 is a graph illustrating a relationship between: a substrate-sidedepth of a depletion layer formed in the amplifying transistor accordingto the first embodiment, and a potential;

FIG. 9A is a graph illustrating a relationship between thesubstrate-side depth and impurity concentration of the amplifyingtransistor according to the first embodiment;

FIG. 9B is a graph illustrating a relationship between an FD voltage anda degree of modulation of the amplifying transistor according to thefirst embodiment;

FIGS. 10A, 10B and 10C is a schematic cross-sectional view illustratinga manufacturing process of the solid-state imaging device according tothe first embodiment;

FIG. 11 is a schematic cross-sectional view of an amplifying transistoraccording to a second embodiment;

FIG. 12 is a graph illustrating a relationship between a substrate-sidedepth and impurity concentration in the amplifying transistor accordingto the second embodiment; and

FIG. 13 is a graph illustrating a relationship between a substrate-sidedepth and impurity concentration of an ideal model according to a thirdembodiment.

DETAILED DESCRIPTION

In general, according to one embodiment, a solid-state imaging device isprovided. The solid-state imaging device includes a photoelectricconversion element, a floating diffusion, and an amplifying transistor.The photoelectric conversion element photoelectrically convert incidentlight into electric charges with an amount corresponding to an amount ofthe incident light, and accumulates the electric charges. The floatingdiffusion accumulates the electric charges read out from thephotoelectric conversion element. The amplifying transistor includes agate electrode connected to the floating diffusion, and outputs a signalbased on the amount of the electric charges accumulated in the floatingdiffusion. The amplifying transistor includes a first concentrationregion disposed in at least a part of the maximum region of thedepletion layer and a second concentration region disposed at a deeperposition than the first concentration region, and has higher impurityconcentration than that of the first concentration region.

Exemplary embodiments of a solid-state imaging device and a method formanufacturing the solid-state imaging device will be explained below indetail with reference to the accompanying drawings. The presentinvention is not limited to the following embodiments.

First Embodiment

FIG. 1 is a block diagram illustrating a schematic configuration of adigital camera 1 with a solid-state imaging device 14 according to afirst embodiment. As illustrated in FIG. 1, the digital camera 1includes a camera module 11 and a latter part processor 12.

The camera module 11 includes an imaging optical system 13 and thesolid-state imaging device 14. The imaging optical system 13 receiveslight from a photographic subject and forms an image of a photographicsubject image. The solid-state imaging device 14 takes the photographicsubject image formed by the imaging optical system 13, and outputs animage signal obtained by taking the image to the latter part processor12. This camera module 11 is applied, for example, to electronic devicessuch as a portable terminal with camera in addition to the digitalcamera 1.

The latter part processor 12 includes an Image Signal Processor (ISP)15, a storage unit 16, and a display unit 17. The ISP 15 performs asignal process of the image signal input from the solid-state imagingdevice 14. This ISP 15 performs, for example, a high quality imageprocess such as a denoising process, a defective pixel correctionprocess, and a resolution conversion process.

Then, the ISP 15 outputs the image signal after the signal process tothe storage unit 16, the display unit 17, and a signal processingcircuit 21, which will be described below (see FIG. 2). The signalprocessing circuit 21 is included in the solid-state imaging device 14in the camera module 11. The image signal fed back from the ISP 15 tothe camera module 11 is used for adjustment and control of thesolid-state imaging device 14.

The storage unit 16 stores the image signal input from the ISP 15 as animage. The storage unit 16 outputs the image signal of the stored imageto the display unit 17 according to, for example, an operation by theuser. The display unit 17 displays an image corresponding to the imagesignal input from the ISP 15 or the storage unit 16. This display unit17 is, for example, a liquid crystal display.

Next, with reference to FIG. 2, the solid-state imaging device 14included in the camera module 11 will be described. FIG. 2 is a blockdiagram illustrating a schematic configuration of the solid-stateimaging device 14 according to the first embodiment. As illustrated inFIG. 2, the solid-state imaging device 14 includes an image sensor 20and the signal processing circuit 21.

The image sensor 20 includes a peripheral circuit 22 and a pixel array23. The peripheral circuit 22 includes a vertical shift register 24, atiming control unit 25, a correlated double sampling unit (CDS) 26, ananalog-to-digital converter (ADC) 27, and a line memory 28.

The pixel array 23 is disposed in an imaging area of the image sensor20. In the pixel array 23, photodiodes as a plurality of photoelectricconversion elements corresponding to respective pixels of the capturedimage are disposed in the horizontal direction (row direction) and thevertical direction (column direction) in a two-dimensional arraypattern. In the pixel array 23, each photoelectric conversion elementcorresponding to each pixel generates a signal charges (for example,electrons) corresponding to an amount of incident light.

Here, a circuit configuration of this pixel array 23 will be describedwith reference to FIG. 3. FIG. 3 is a diagram illustrating one exampleof the circuit configuration of the pixel array 23 according to thefirst embodiment. The circuit illustrated in FIG. 3 is a circuitselectively extracted from a part corresponding to two pixels of thecaptured image in the pixel array 23.

As illustrated in FIG. 3, the pixel array 23 includes photoelectricconversion elements PD, read-out transistors TR, and a floatingdiffusion FD. The pixel array 23 includes a reset transistor RST, anaddress transistor ADR, and an amplifying transistor AMP.

The photoelectric conversion element PD photoelectrically convertsincident light into electric charges of an amount corresponding to anamount of the incidest light, and accumulates the electric charges. Inthe photoelectric conversion element PD, a cathode is connected to theground, and an anode is connected to a source of the read-out transistorTR.

The read-out transistor TR is disposed corresponding to thephotoelectric conversion element PD, and reads out the signal chargesaccumulated in the photoelectric conversion element PD so as to transferthe signal charges to the floating diffusion FD in the case where a gateelectrode of the read-out transistor TR receives a transfer signal. Adrain of the read-out transistor TR is connected to the floatingdiffusion FD.

The floating diffusion FD temporarily accumulates the signal chargestransferred from the read-out transistor TR. This floating diffusion FDis connected to a source of the reset transistor RST.

A drain of the reset transistor RST is connected to a power supplyvoltage line Vdd. This reset transistor RST resets the electricpotential of the floating diffusion FD to the electric potential of thepower supply voltage in the case where a gate electrode of the resettransistor RST receives a reset signal before the signal charges istransferred to the floating diffusion FD.

The floating diffusion FD is connected to a gate electrode of theamplifying transistor AMP. A source of the amplifying transistor AMP isconnected to a signal line for outputting the signal charges to theperipheral circuit 22. A drain of the amplifying transistor AMP isconnected to a source of the address transistor ADR. A drain of theaddress transistor ADR is connected to the power supply voltage lineVdd.

In the pixel array 23, in the case where an address signal is input intoa gate electrode of the address transistor ADR, the amplifyingtransistor AMP amplifies an image signal VSIG in accordance with avoltage of the floating diffusion FD (hereinafter, “FD voltage”) thatvaries depending on an amount of signal charges acculated in thefloating diffusion FD, and outputs the amplified image signal VSIG tothe peripheral circuit 22.

Thus, in the solid-state imaging device 14, the amplifying transistorAMP changes the image signal VSIG corresponding to the FD voltage, whichis changed corresponding to the amount of the signal charges generatedin the photoelectric conversion element PD, so as to obtain the imagesignal corresponding to the photographic subject image. Therefore, theoutput of the amplifying transistor AMP is preferred to be changedlinearly with respect to the FD voltage in order to obtain anappropriate image signal corresponding to the photographic subjectimage.

However, a conventional amplifying transistor has a tendency that outputlinearity to the FD voltage is deteriorated in the case where thephotographic subject is comparatively bright (hereinafter, “in thebright situation”) compared with the case where the photographic subjectis comparatively dark (hereinafter, “in the dark situation”).

This point will be described with reference to FIG. 4A and FIG. 4B. FIG.4A and FIG. 4B are graphs illustrating output characteristics of theconventional amplifying transistor.

Here, FIG. 4A illustrates a relationship between a voltage of the imagesignal VSIG output from the conventional amplifying transistor(hereinafter described as a “VSIG voltage”) and the FD voltage. In FIG.4A, an ideal output characteristic L1 and an actual outputcharacteristic L2 are respectively illustrated by a dashed line and asolid line. FIG. 4B illustrates a relationship between: respectivedifferential values L1′ and L2′ of the output characteristics L1 and L2illustrated in FIG. 4A (ΔVSIG voltage/ΔFD voltage: hereinafter describedas a “degree of modulation”); and the FD voltage.

As illustrated in FIG. 4A, the output characteristic L2 of theconventional amplifying transistor is a characteristic close to theideal output characteristic L1 in the dark situation. That is, theconventional amplifying transistor outputs an appropriate VSIG voltagecorresponding to the FD voltage in the dark situation.

On the other hand, the output characteristic L2 of the conventionalamplifying transistor is found to be further away from the ideal outputcharacteristic L1 as the photographic subject becomes brighter.Specifically, the conventional amplifying transistor is prone to outputa higher VSIG voltage than the ideal VSIG voltage as the photographicsubject becomes brighter. As a result, as illustrated in FIG. 4B, theconventional amplifying transistor provides a lower degree of modulationas the photographic subject becomes brighter.

Thus, in the conventional amplifying transistor, non-linearity is likelyto appear in the output characteristic in the bright situation. Thecause of this non-linearity will be specifically described withreference to FIG. 5A to FIG. 5C. FIG. 5A and FIG. 5B are schematiccross-sectional views of the conventional amplifying transistor. FIG. 5Aillustrates a schematic cross-sectional view in the dark situation. FIG.5B illustrates a schematic cross-sectional view in the bright situation.FIG. 5C is a graph illustrating a relationship between a substrate-sidedepth of a depletion layer formed in the conventional amplifyingtransistor and a potential (a channel surface potential just under agate electrode).

As illustrated in FIG. 5A, a conventional amplifying transistor AMP_0includes a gate electrode G connected to the floating diffusion. Theamplifying transistor AMP_0 includes a source S and a drain D in a wellregion I of a semiconductor substrate.

In the amplifying transistor AMP_0, a depletion layer B is formed on thesubstrate surface between the source S and the drain D. The depletionlayer B is an electrically insulated region without any carrier.

The FD voltage in the dark situation is higher than that in the brightsituation. Accordingly, a difference in electric potential between thegate electrode G and the substrate is large. The depletion layer Blargely extends from the surface of the well region I in a direction ofthe substrate-side depth (see Ld and Dd in FIG. 5C).

Here, assuming that the capacitance of the gate electrode G is CG andthe capacitance of the depletion layer B is CB, the degree of modulation(ΔVSIG voltage/ΔFD voltage) is expressed by CG/(CG∥CB). The capacitanceCB of the depletion layer B is inversely proportional to the width ofthe depletion layer B. Accordingly, the capacitance CB of the depletionlayer B decreases as the depletion layer B extends further. The decreasein CB increases the degree of modulation. Thus, the degree of modulationbecomes higher in the dark situation (see P1 in FIG. 4B).

On the other hand, as illustrated in FIG. 5B, the FD voltage becomeslower in the bright situation compared with that in the dark situation.Accordingly, the difference in electric potential between the gateelectrode G and the substrate is small compared with that in the darksituation. The depletion layer B becomes narrower compared with that inthe dark situation (see L1 and D1 in FIG. 5C). Then, the capacitance CBof the depletion layer B becomes larger as the width of the depletionlayer B becomes narrower. Accordingly, the degree of modulation becomeslower. Therefore, the degree of modulation becomes lower in the brightsituation (see P2 in FIG. 4B).

Thus, in the conventional amplifying transistor, non-linearity appearsin the output characteristic because the width of the depletion layer Bin the dark situation is different from the width of the depletion layerin the bright situation.

In the solid-state imaging device 14 according to the first embodiment,the variation in the width of the depletion layer B in the darksituation and in the bright situation is minimized so as to obtain theamplifying transistor AMP with a satisfactory output linearity.

A configuration of this amplifying transistor AMP will be described withreference to FIG. 6 to FIG. 8. FIG. 6 is a schematic cross-sectionalview of the pixel array 23 according to the first embodiment. FIG. 7 isa schematic cross-sectional view of the amplifying transistor AMPaccording to the first embodiment. FIG. 8 is a graph illustrating arelationship between: the substrate-side depth of the depletion layerformed in the amplifying transistor AMP according to the firstembodiment, and the potential.

FIG. 6 schematically illustrates a cross section of a portion includingthe photoelectric conversion element PD, the read-out transistor TR, thefloating diffusion FD, the reset transistor RST, and the amplifyingtransistor AMP. Here, while the illustration is omitted, a supportingsubstrate is disposed at the lower layer side of the structureillustrated in FIG. 6 via an interlayer insulating film where amultilayer interconnection layer is formed. In the upper layer of thestructure illustrated in FIG. 6, a color filter is disposed. The colorfilter selectively transmits incident light of a predetermined colorthrough the interlayer insulating film. On the top surface of the colorfilter, a microlens is disposed. The microlens condenses incident lightto a light receiving portion of the photoelectric conversion element PD.

As illustrated in FIG. 6, the pixel array 23 includes a semiconductorsubstrate 100 where a P-type well layer 92 is disposed on a SUB layer 91of a first conductivity type (hereinafter described as “N type”) or asecond conductivity type (hereinafter described as “P type”).

The pixel array 23 includes the photoelectric conversion element PD. Thephotoelectric conversion element PD is formed of, for example, a P-typesemiconductor region and an N-type the semiconductor region. The P-typesemiconductor region extends from the top surface of the semiconductorsubstrate 100 along the depth direction of the semiconductor substrate100. The N-type semiconductor region extends from the inferior surfaceof the P-type semiconductor region along the depth direction of thesemiconductor substrate 100.

The pixel array 23 includes a gate electrode 31 of the read-outtransistor TR, a gate electrode 41 of the reset transistor RST, and agate electrode 51 of the amplifying transistor AMP. These gateelectrodes 31, 41, and 51 are disposed on the semiconductor substrate100 via the respective gate oxide films 32, 42, and 52. A side wall 94is disposed on each side surface of the gate electrodes 31, 41, and 51.

The floating diffusion FD doped with N-type impurity is disposed insideof the semiconductor substrate 100 between the gate electrode 31 of theread-out transistor TR and the gate electrode 41 of the reset transistorRST.

Inside of the semiconductor substrate 100, semiconductor regions dopedwith N-type impurity are disposed in predetermined respective positionsfor a drain 43 of the reset transistor RST and a source 53 and a drain54 of the amplifying transistor AMP. The photoelectric conversionelement PD, the read-out transistor TR, and the reset transistor RST areelectrically element-isolated from the amplifying transistor AMP by anSTI 93.

FIG. 7 illustrates a schematic enlarged view of the amplifyingtransistor AMP illustrated in FIG. 6. As illustrated in FIG. 7, theamplifying transistor AMP according to the first embodiment includes afirst concentration region 56 and a second concentration region 57 inthe well layer 92. The first concentration region 56 has a low impurityconcentration. The second concentration region 57 has a higher impurityconcentration compared with the first concentration region 56.

The first concentration region 56 is disposed in at least a part of amaximum region 55 of the depletion layer formed between the source 53and the drain 54. The maximum region 55 of the depletion layer(hereinafter described simply as “the maximum region 55”) is the maximumregion within which the depletion layer can extend. Specifically, themaximum region 55 is a region of the depletion layer formed in the casewhere the photoelectric conversion element PD is not receiving light.

The depletion layer is likely to extend more as the well layer 92 haslower impurity concentration. Accordingly, disposing the firstconcentration region 56 with low impurity concentration in the maximumregion 55 allows the depletion layer to extend largely in the same wayas in the dark situation where the depletion layer extends readily, evenin the bright situation where the depletion layer does not extendreadily.

The width of the maximum region 55, that is, the maximum depth of thedepletion layer is, for example, 0.2 μm that is approximately the samemaximum depth (width) of the source 53 and the drain 54. The depletionlayer can be formed up to a deeper position than the maximum depth ofthe source 53 and the drain 54. The first concentration region 56 isdisposed in a shallower position than the maximum depth of the source 53and the drain 54.

The second concentration region 57 is disposed adjacent to the firstconcentration region 56 in a deeper position than the firstconcentration region 56. The depletion layer has difficulty in extendingas the well layer 92 has higher impurity concentration. Accordingly, thesecond concentration region 57 with higher impurity concentration thanthat of the first concentration region 56 is disposed at a lower side ofthe first concentration region 56, so as to reduce extension of thedepletion layer.

Thus, the amplifying transistor AMP according to the first embodimentfacilitates the extension of the depletion layer in the firstconcentration region 56 while reducing the extension of the depletionlayer in the second concentration region 57. This reduces the differencebetween: the width of the depletion layer formed in the dark situation;and the width of the depletion layer formed in the bright situation (seeDd and D1 in FIG. 8). The amplifying transistor AMP according to thefirst embodiment improves the output linearity compared with theconventional amplifying transistor AMP_0.

Next, a description will be specifically given of a relationship betweenthe substrate-side depth and the impurity concentration of theamplifying transistor AMP according to the first embodiment withreference to FIG. 9A and FIG. 9B. FIG. 9A is a graph illustrating therelationship between the substrate-side depth and the impurityconcentration. FIG. 9B is a graph illustrating a relationship betweenthe FD voltage and the degree of modulation.

Here, FIG. 9A illustrates a relationship between the substrate-sidedepth and the impurity concentration of the conventional amplifyingtransistor AMP_0 by a one-dot chain line, and illustrates a relationshipbetween the substrate-side depth and the impurity concentration of theamplifying transistor AMP according to the first embodiment by a dashedline.

In FIG. 9A, a solid line indicates an ideal model AMP_I where a regionwith constant impurity concentration of 1.00E+16 (/cm3) and a regionwith constant impurity concentration of 1.00E+18 (/cm3) are adjacent toeach other at a substrate-side depth of 0.2 μm.

The first concentration region 56 and the second concentration region 57of the amplifying transistor AMP according to the first embodiment areformed based on the ideal model AMP_I illustrated in FIG. 9A. Forexample, impurity such as boron is implanted with an implantation energyof 120 KeV in a position at the substrate-side depth of about 0.45 μm.Subsequently, an annealing process is performed so as to form the secondconcentration region 57 with high impurity concentration.

The impurity implanted into the well layer 92 is diffused by theabove-described annealing process. Accordingly, the first concentrationregion 56 with lower impurity concentration than that of the secondconcentration region 57 is formed in a shallower position than thesecond concentration region 57. The impurity concentration of the firstconcentration region 56 is equal to or less than at least ½ of theimpurity concentration of the second concentration region 57.

On the other hand, the conventional amplifying transistor AMP_0 isformed with constant impurity concentration in the substrate-side depth.For example, impurity such as boron is implanted with respectiveimplantation energies of 80 KeV and 120 KeV in positions at thesubstrate-side depths of about 0.08 μm and about 0.28 μm. Subsequently,an annealing process is performed so as to form regions with roughlyuniform impurity concentrations.

Thus, in the conventional amplifying transistor AMP_0, the well layer isformed with constant impurity concentration. Therefore, a difference inwidth of the depletion layer occurs between in the dark situation and inthe bright situation. This caused the low output linearity. Asillustrated in FIG. 9B, it is seen that the degree of modulation of theconventional amplifying transistor AMP_0 has a large gradient comparedwith the ideal model AMP_I, thus providing low linearity of the degreeof modulation.

In contrast, it is seen that the degree of modulation of the amplifyingtransistor AMP according to the first embodiment has a small gradientcompared with the conventional amplifying transistor AMP_0, thusproviding improved linearity of the degree of modulation compared withthe conventional amplifying transistor AMP_0.

As described above, with the amplifying transistor AMP according to thefirst embodiment, the second concentration region 57 and the firstconcentration region 56 with lower impurity concentration than that ofthe second concentration region 57 are disposed. This reduces thedifference in width of the depletion layer between in the dark situationand in the bright situation, thus improving the output linearity.

Next, a description will be given of a method for manufacturing thesolid-state imaging device 14 according to the first embodiment withreference to FIG. 10. FIG. 10 is a schematic cross-sectional viewillustrating a manufacturing process of the solid-state imaging device14 according to the first embodiment. A method for manufacturing aportion other than the pixel array 23 in the solid-state imaging device14 is similar to that of a typical CMOS image sensor. Therefore, adescription will be given of a method for manufacturing the pixel array23 portion in the solid-state imaging device 14 below.

As illustrated in FIG. 10( a), in the case where the pixel array 23 ismanufactured, the silicon semiconductor substrate 100 where the N-typeor P-type SUB layer 91 and the P-type well layer 92 are laminated isprepared.

Here, although the illustration is omitted, in a subsequent step, theinterlayer insulating film that includes the multilayer interconnectionlayer is formed on the upper layer of the semiconductor substrate 100.The supporting substrate is laminated on the upper layer of theinterlayer insulating film. Subsequently, in a state where thesupporting substrate is supported, the surface on the opposite side ofthe surface on which the supporting substrate of the semiconductorsubstrate 100 is laminated is ground so as to expose the surface of thewell layer 92.

The STI 93 is formed to reach a predetermined depth from the surface ofthe well layer 92 at a formation position of an element isolation areain the well layer 92. Specifically, a groove (a trench) is formed toreach the predetermined depth from the surface of the well layer 92 atthe formation position of the STI 93 in the well layer 92. Subsequently,a silicon oxide film is formed inside of the trench by, for example,Chemical Vapor Deposition (CVD) so as to form the STI 93.

Here, while the STI 93 is disposed as the element isolation area, theelement isolation area may be formed by Local Oxidation Of Silicon(LOCOS). For example, the element isolation area may be formed by ionimplantation of oxygen into the formation position of the elementisolation area in the semiconductor substrate 100 and then performing anannealing process.

Subsequently, as illustrated in FIG. 10( b), the photoelectricconversion element PD is formed in the well layer 92. This photoelectricconversion element PD is formed by, for example, sequential ionimplantations of N-type impurity and P-type impurity from apredetermined position on the top surface of the semiconductor substrate100 and then performing an annealing process.

As illustrated in FIG. 10( b), the gate oxide films 32, 42, and 52 eachformed of silicon oxide are formed on the top surface of the well layer92 corresponding to the respective formation positions of the read-outtransistor TR, the reset transistor RST, and the amplifying transistorAMP. Furthermore, the gate electrodes 31, 41, and 51 each formed ofpolysilicon are formed on the respective gate oxide films 32, 42, and52.

Here, for example, a silicon oxide film and a polysilicon layer withrespective predetermined film thicknesses are sequentially formed on theoverall top surface of the well layer 92. Subsequently, a resist isformed by photolithography in order to selectively coat the top surfaceof the polysilicon layer in respective positions where the gateelectrodes 31, 41, and 51 are to be formed.

Dry etching is performed using the resist as a mask, so as to remove thepolysilicon layer and the silicon oxide film in the unnecessary portion,thus simultaneously forming the gate oxide films 32, 42, and 52 and thegate electrodes 31, 41, and 51.

Subsequently, as illustrated in FIG. 10( b), an LDD diffusion layer 80is formed such that a desired pattern is formed by application of theresist and then N-type impurity such as arsenic and phosphorus isimplanted by ion implantation into the respective formation positionsfor the floating diffusion FD, the drain 43 of the reset transistor RST,and the source 53 and the drain 54 of the amplifying transistor AMP.

Subsequently, as illustrated in FIG. 10( c), the side wall 94 is formedby a known etchback on each side surface of the gate electrodes 31, 41,and 51. For example, a silicon nitride film is formed to cover each sidesurface of the gate electrodes 31, 41, and 51. Subsequently, a siliconoxide layer is formed on the overall top surface of the semiconductorsubstrate 100. The side wall 94 is formed by etchback of this siliconoxide layer using anisotropy plasma etching.

The N-type impurity implanted by ion implantation into the respectiveformation positions for the floating diffusion FD, the drain 43 of thereset transistor RST, and the source 53 and the drain 54 of theamplifying transistor AMP is activated by an annealing process, so as toform the floating diffusion FD, the drain 43 of the reset transistorRST, and the source 53 and the drain 54 of the amplifying transistorAMP.

The first concentration region 56 and the second concentration region 57are formed inside the well layer 92 corresponding to the formationposition of the amplifying transistor AMP. The first concentrationregion 56 and the second concentration region 57 are formed throughwhich impurity is implanted at a deeper position than the position wherethe first concentration region 56 is formed and then an annealingprocess is performed, as described above. Thus, the first concentrationregion 56 and the second concentration region 57 have impurityconcentrations different from those of the read-out transistor TR andthe reset transistor RST.

Subsequently, the interlayer insulating film, the color filter, and themicrolens are sequentially formed on the structure illustrated in FIG.10( c). Thus, the pixel array 23 is manufactured.

As described above, in the solid-state imaging device 14 according tothe first embodiment, the well layer 92 corresponding to the formationposition of the amplifying transistor AMP includes the firstconcentration region 56 with low impurity concentration and the secondconcentration region 57 with higher impurity concentration than that ofthe first concentration region 56. The first concentration region 56 isdisposed at least in a part of the maximum region 55 of the depletionlayer. The second concentration region 57 is disposed in the deeperposition than the first concentration region 56.

This ensures a small difference between: the width of the depletionlayer formed in the dark situation where the FD voltage is high, and thewidth of the depletion layer formed in the bright situation where the FDvoltage is low, thus improving the output linearity compared with theconventional amplifying transistor AMP_0.

While in the above-described first embodiment the example where thefirst concentration region 56 and the second concentration region 57 areformed after the gate electrode 51 is formed has been described, thefirst concentration region 56 and the second concentration region 57 maybe formed before forming the gate electrode 51.

Second Embodiment

The amplifying transistor AMP may include a third concentration regionwith higher impurity concentration than that of the first concentrationregion 56 in a region at a shallower position than the firstconcentration region 56.

Hereinafter, this point will be described with reference to FIG. 11 andFIG. 12. FIG. 11 is a schematic cross-sectional view of an amplifyingtransistor according to the second embodiment. FIG. 12 is a graphillustrating a relationship between a substrate-side depth and impurityconcentration in the amplifying transistor according to the secondembodiment.

As illustrated in FIG. 11 and FIG. 12, an amplifying transistor AMP′according to the second embodiment includes a third concentration region58 in a region at a shallower position than the first concentrationregion 56. This third concentration region 58 functions as a shielddiffusion layer that prevents reception of noise signals due to theinterface state of the semiconductor substrate 100 interface.

The third concentration region 58 is formed, similarly to the firstconcentration region 56 and the second concentration region 57, suchthat impurity such as boron is implanted by ion implantation into thewell layer 92 and then an annealing process is performed. As describedabove, the amplifying transistor AMP′ may include the thirdconcentration region 58.

Third Embodiment

In each embodiment described above, the example where the firstconcentration region 56 and the second concentration region 57 areformed based on the ideal model AMP, in which the impurity concentrationis changed at the position of 0.2 μm, has been described. However, thefirst concentration region 56 and the second concentration region 57 maybe formed based on the ideal model other than the ideal model AMP_I.This point will be described with reference to FIG. 13. FIG. 13 is agraph illustrating a relationship between a substrate-side depth andimpurity concentration of the ideal model according to a thirdembodiment.

As illustrated in FIG. 13, in the ideal model AMP_I′, a region withconstant impurity concentration of 1.00E+16 (/cm3) and a region withconstant impurity concentration of 1.00E+18 (/cm3) are adjacent to eachother at a substrate-side depth of 0.25 μm.

In the amplifying transistor according to the third embodiment, thefirst concentration region 56 and the second concentration region 57 areformed based on this ideal model AMP_I′. Accordingly, the firstconcentration region 56 is formed across the region at a deeper positionthan 0.2 μm, which is the maximum depth of the depletion layer, that is,across the region at a deeper position than the region where the source53 and the drain 54 are formed. Additionally, the second concentrationregion 57 is formed at the deeper position than the formation positionof the second concentration region 57 in each embodiment describedabove.

As described above, the first concentration region 56 may be formedacross the region at a deeper position than the maximum region 55 of thedepletion layer and the region where the source 53 and the drain 54 areformed. Alternatively, the first concentration region 56 may be formedin the region at a deeper position than the region where the source 53and the drain 54 are formed.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andspirit of the inventions.

What is claimed is:
 1. A solid-state imaging device comprising: aphotoelectric conversion element configured to photoelectrically convertincident light into electric charges with an amount corresponding to anamount of the incident light and accumulate the electric charges; afloating diffusion configured to accumulate the electric charges readout from the photoelectric conversion element; and an amplifyingtransistor with a gate electrode connected to the floating diffusion,the amplifying transistor being configured to output a signal based onthe amount of the electric charges accumulated in the floatingdiffusion, wherein the amplifying transistor includes: a source; adrain; a first concentration region disposed in at least a part of amaximum region of a depletion layer; and a second concentration regiondisposed at a deeper position than the first concentration region, thesecond concentration region having higher impurity concentration thanthat of the first concentration region, and a depth of the firstconcentration region is shallower than those of the source and the drainof the amplifying transistor.
 2. The solid-state imaging deviceaccording to claim 1, wherein the first concentration region is disposedacross the maximum region of the depletion layer and a region at adeeper position than respective maximum depths of a source and a drainof the amplifying transistor.
 3. The solid-state imaging deviceaccording to claim 1, wherein the amplifying transistor includes a thirdconcentration region with higher impurity concentration than theimpurity concentration of the first concentration region in a region ata shallower position than the first concentration region.
 4. Thesolid-state imaging device according to claim 1, wherein the firstconcentration region has impurity concentration equal to or less than ½of the impurity concentration of the second concentration region.
 5. Thesolid-state imaging device according to claim 1, wherein the firstconcentration region and the second concentration region are disposedadjacent to each other.
 6. The solid-state imaging device according toclaim 1, further comprising: a read-out transistor that reads out theelectric charges accumulated in the photoelectric conversion element;and a reset transistor that resets the electric charges accumulated inthe floating diffusion, wherein the first and second concentrationregions have impurity concentrations different from those of theread-out and reset transistors.
 7. The solid-state imaging deviceaccording to claim 1, wherein a depth of the second concentration regionis deeper than that of an element isolation area.